- Upper Detector Layer: ISGRI
Cadmium Telluride (CdTe) is a semiconductor operating at ambient
C is the optimum range. With their small area,
the CdTe detectors are ideally suited to build an image
with good spatial resolution.
The CdTe layer is made of 8 identical Modular Detection Units
each having 32
64 pixels (see Figure ). Total sensitive area of the detector is 2621 cm
- Lower Detector Layer: PICsIT
Caesium Iodide is a scintillation crystal. The CsI(Tl) layer is divided
into eight rectangular modules
32 detector elements (see Figure ). In each module, there are two
independent semi-module each one with its independent Front End Electronics.
Total sensitive area of the detector is 2994 cm
- Noisy Pixels
It is possible that with the time some of the pixels of the detector may become out of order
and start to produce outputs not triggered by an income photon, i.e., to become ``noisy''.
If the particular pixel countrate is too high relatively to the module countrate,
then the on-board
electronics switch it off. In ISGRI, the noisy pixels can recover after being
switched off for some time and disabled pixels are periodically reset to check
Schematic view of the PICsIT layer.
Each module number is indicated. The dotted lines represent
the division in semimodules whose number is indicated at the top.
The black pixels are the dead ones. The (
coordinates are the IBIS ones both ranging from 0 to 63.
X-axis is directed toward the source located above the page.
-axis is pointing positively to the sun.
In PICsIT, pixels cannot be recovered so easily. PICsIT pixel will remain off
once killed. Only if half of the detector (or so) will be off, an attempt will be
made to turn pixels on.
The current situation is shown on Figure .
Overall the killed pixels are less than 1%.